1. Field of the Invention
This invention generally relates to heat treating furnaces, and more particularly to a sheet material heating furnace having a slot extending the length of the roof through which tongs supporting the sheet material move, and a seal for the slot.
2. Description of the Prior Art
It is common practice in the heating of sheet material to suspend the sheet from one of its edges in a vertical plane so that both of its surfaces may be heated, and move the sheet through the heating chamber of a furnace. For example, in the tempering of glass sheets, the sheets are ordinarily suspended in a vertical plane from their upper edges by tongs and passed horizontally through a tunnel-type furnace wherein the glass sheets are heated to a temperature approximating their softening point.
Furnaces employed in the above-mentioned process usually are of the type having an elongated heating chamber wherein the roof is provided with a slot extending lengthwise of the furnace and opening into opposed entrance and exit openings provided in the furnace walls. Such furnaces, which may be of either the straight-through or the circular type, generally have a track suspended above the slot and outwardly of the roof on which a carrier is movably supported. The carrier normally includes hanger rods which project through the slot for supporting a horizontally disposed supporting bar from which tongs are hung to suspend the sheets within the heating chamber of the furnace. Examples of such prior art furnace structures are disclosed in U.S. Pat. Nos. 2,115,106, issued on Apr. 26, 1938; 2,197,550, issued on Apr. 16, 1940; 2,272,966, issued on Feb. 10, 1942 and 3,957,479, issued on May 18, 1976.
In the heating of sheet materials in the above-described furnaces, difficulty has been encountered in achieving uniform temperature in the sheets inasmuch as different portions thereof may be exposed to uneven temperatures due to the so-called "chimney effect" created by the open slot, which produces a variable flow of hot gases throughout the heating chamber. The uneven temperatures consequently resulting in different portions of the sheets produce stresses therein causing warpage and/or in the case of glass, breakage of the sheet material.
Efforts have been made in the past to prevent the setting up of these undesirable temperature conditions in the heating chamber, and also to prevent undue loss of heat therefrom, by closing or sealing the slot in the roof of the furnace, but heretofore no entirely satisfactory closure has been found. Attempts have been made to seal the slot by closures which open up as the sheet traverses the furnace. For example, in the aforementioned United States patents, U.S. Pat. No. 2,115,106 discloses a slot in the furnace roof which is closed by a pair of asbestos strips while in U.S. Pat. No. 2,197,550, two series of transversely extending asbestos fingers are pivotally mounted on the roof of the furnace over the slot with their inner ends engaging each other. However, in both cases the slot is open to the environment surrounding the furnace as the carrier apparatus moves along the slot.
Also, attempts have been made to provide the slot with closures which do not open as a carrier traverses the slot. In the previously mentioned United States patents, U.S. Pat. No. 2,272,966 shows the slot to be closed by a series of mechanical plates which move with the carrier apparatus and in U.S. Pat. No. 3,957,479, the slot is closed by a housing which surrounds and overlies the slot and carrier apparatus and wherein cooling air is admitted to the housing for cooling the bearings employed in the carrier. However, the structures of these closures are complex, expensive to manufacture and difficult to maintain.